Ion current density measuring method and instrument, and semiconductor device manufacturing method

ABSTRACT

A wafer is exposed to a plasma. Here, the wafer includes a semiconductor or a conductor  1  provided on an insulator  6 , an insulator  2  formed thereon and having a region the thickness of which has been made locally thin, and a 2nd conductor  4  provided on the insulator  2 , one of the semiconductor or the conductor  1  and the 2nd conductor  4  having a 1st region from the surface of which a substantially total solid angle is formed, the other having a 2nd region a solid angle formed from the surface of which is made smaller than the 1st region. Then, a voltage is applied to the semiconductor or the conductor  1  and the 2nd conductor  4  so as to measure a time elapsing until the insulator  2  undergoes a dielectric breakdown. Moreover, the ion current density is determined from an electric charge required therefor and an area exposed onto the surface of the 2nd conductor  4.    
     Consequently, it becomes possible to measure, on the wafer, the current density of the ions launched into the wafer, thereby allowing the measuring method of the icon current density to be made suitable for the mass production.

TECHNICAL FIELD

The present invention is suitable for an ion current density measuringmethod and the measuring instrument, and a semiconductor devicemanufacturing method. Here, the method and the instrument are the onesfor measuring the current density of ions launched into a wafer and itsdistribution, which are one of the plasma characteristics in the plasmaprocessing for an etching, a CVD, or the like.

BACKGROUND ART

In the manufacturing of a semiconductor device, and in aplasma-utilizing wafer processing apparatus in particular, the currentdensity of the ions launched into the wafer and its distribution areimportant factors for determining the properties of the processing,e.g., the rate and the uniformity of the etching or the deposition, anddamages of the components.

As a method of measuring the current density of the ions, there has beenknown a technology referred to as a probe method. This method has beendescribed in, e.g., Shinriki Teii, “Basic Plasma Engineering”, 1st ed.(published on May 30, 1986), Chap. 3. In the probe method, a probe isinserted into a region where the ion current density is to be measured,and positively charged ions are led selectively into a measuring unit byan electric voltage applied to the probe, thereby measuring the ioncurrent density.

Also, concerning a technology of making a contrivance to the wafer so asto measure the ion current, JP-A-8-213374 can be cited, for example.

Although the above-described technology has an advantage of making itpossible to measure the ion current density at an arbitrary positionwithin the plasma, the ion current density measurement by the insertionof the probe was a difficult task. This is mainly because, in a plasmaprocessing apparatus used for the mass production of the semiconductordevices, there exists a fear that the probe insertion causes metalcontamination or foreign substances to be produced, or, it is requiredto provide a mass-producing apparatus with a port for the electricalwiring.

In the technology disclosed in JP-A-8-213374, in principle, theabove-described probe is built and incorporated in the wafer. Thiscondition requires that the electrical wiring for the voltage control orthe signal fetching be set from the wafer to the outside of the plasmaprocessing apparatus. Accordingly, the technology cannot be said to beappropriate for the mass-producing apparatus.

The object of the present invention is to measure the current density ofthe ions launched into the wafer without installing the electricalwiring through the plasma processing apparatus or without making thespecial contrivance to the wafer or a wafer-supporting member, therebyproviding the ion current density measuring method and the measuringinstrument and the semiconductor device manufacturing method which aresuitable for the mass production.

DISCLOSURE OF INVENTION

In order to accomplish the above-described objects, in the presentinvention, there is provided the wafer including a semiconductor or aconductor provided on an insulator, an insulator formed on thesemiconductor or the conductor and having a region the thickness ofwhich has been made locally thin, and a 2nd conductor provided on theinsulator, one of the semiconductor or the conductor and the 2ndconductor having a 1st region from the surface of which a substantiallyentire solid angle is formed, the other having a 2nd region a solidangle formed from the surface of which is made smaller than the 1stregion, wherein the wafer is exposed to the plasma, and a voltage isapplied to the semiconductor or the conductor and the 2nd conductor soas to measure a time that will elapse until the insulator undergoes adielectric breakdown, then determining the ion current density from acharge and an area exposed onto the surface of the 2nd conductor, thecharge being needed to cause the insulator to undergo the dielectricbreakdown in correspondence with the voltage.

On account of this, one of the semiconductor or the conductor and the2nd conductor has the 1st region from the surface of which thesubstantially total solid angle is formed, and the other has the 2ndregion the solid angle formed from the surface of which is made smallerthan the 1st region. As a result, electrons and the ions reach the 1stregion by the same flux on average. Meanwhile, in the 2nd region, theflux of the high kinetic-energy ions exhibits an isotropic behavior,thus becoming larger than the flux of the low kinetic-energy electrons.This makes the electric potential of the 2nd conductor positive withreference to that of the semiconductor or the conductor, causing anelectric current to flow through the region in the insulator thethickness of which has been made locally thin. In addition, the wafer isexposed to the plasma, and the voltage is applied to the semiconductoror the conductor and the 2nd conductor so as to measure the time thatwill elapse until the insulator undergoes the dielectric breakdown.This, further, allows the ion current density of the plasma to bedetermined from the charge needed to cause the insulator to undergo thedielectric breakdown and the area exposed onto the surface of the 2ndconductor. Consequently, it becomes possible to measure, on the wafer,the current density of the ions launched into the wafer without givingthe special contrivance to the wafer or the wafer-supporting member,thereby allowing the measuring method of the ion current density to bemade suitable for the mass production.

Also, in the present invention, there is provided the wafer including asemiconductor or a conductor provided on an insulator, an insulatorformed on the semiconductor or the conductor and having a region thethickness of which has been made locally thin, and a 2nd conductorprovided on the insulator, one of the semiconductor or the conductor andthe 2nd conductor having a 1st region from the surface of which asubstantially total solid angle is formed, the other of thesemiconductor or the conductor and the 2nd conductor having a 2nd regiona solid angle formed from the surface of which is made smaller than the1st region, wherein, after the wafer has been exposed to the plasma fora fixed time, a voltage is applied to the semiconductor or the conductorand the 2nd conductor in a state of being not exposed to the plasma,thereby determining a charge that causes the insulator to undergo adielectric breakdown, and then determining the ion current density fromthe charge and an area exposed onto the surface of the 2nd conductor,the charge being needed to cause the insulator to undergo the dielectricbreakdown in correspondence with the voltage.

Moreover, in the present invention, there is provided the waferincluding a semiconductor or a conductor provided on an insulator, aninsulator formed on the semiconductor or the conductor and having aregion the thickness of which has been made locally thin, and a 2ndconductor provided on the insulator, one of the semiconductor or theconductor and the 2nd conductor having a 1st region from the surface ofwhich a substantially total solid angle is formed, the other of thesemiconductor or the conductor and the 2nd conductor having a 2nd regiona solid angle formed from the surface of which is made smaller than the1st region, wherein, after the wafer has been exposed to the plasma fora fixed time, the capacitance-to-voltage characteristic of thesemiconductor or the conductor and the 2nd conductor is measured, and anelectric current flowing through the region that has been made locallythin is calculated from the capacitance-to-voltage characteristic thatwas measured before the wafer has been exposed to the plasma, thendetermining the ion current density from an area exposed onto thesurface of the 2nd conductor.

In addition, in the present invention, there is provided an ion currentdensity measuring instrument for setting a wafer at a predeterminedposition in a plasma processing apparatus so as to measure the ioncurrent density at the time when the wafer is exposed to the plasma, theinstrument having the wafer including a semiconductor or a conductorprovided on an insulator, an insulator formed on the semiconductor orthe conductor and having a region the thickness of which has been madelocally thin, and a 2nd conductor provided on the insulator, one of thesemiconductor or the conductor and the 2nd conductor having a 1st regionfrom the surface of which a substantially total solid angle is formed,the other having a 2nd region a solid angle formed from the surface ofwhich is made smaller than the 1st region, wherein an electric currentflowing through the region in the insulator the thickness of which hasbeen made locally thin is measured, thereby determining the ion currentdensity.

Furthermore, in the present invention, an ion current densitydistribution in the plasma processing apparatus is measured so as toascertain whether or not the distribution measured is in compliance withan ion current density distribution that becomes a criterion, thenmanufacturing the semiconductor devices.

On account of this, when manufacturing the semiconductor devices, theion current density distribution is measured so as to ascertain whetheror not the distribution measured is in compliance with the ion currentdensity distribution that becomes the criterion. Accordingly, in themass production of the semiconductor devices, there exists no necessityfor destroying the mass-produced products to check them.

Still further, in the present invention, there is provided a waferincluding a semiconductor or a conductor, an insulator formed on thesemiconductor or the conductor and having a region the thickness ofwhich has been made locally thin, and a 2nd conductor provided on theinsulator, one of the semiconductor or the conductor and the 2ndconductor having a 2nd region a solid angle formed from the surface ofwhich is made smaller than another region, wherein the wafer is exposedto a plasma, thereby measuring the ion current density of the plasma soas to manufacture the semiconductor devices.

Even further, in the present invention, there are formed on a wafer a1st region into which ions and electrons are launched and a 2nd regioninto which the ions are launched but the electrons are not, wherein thewafer is exposed to a plasma, then measuring the ion current density ofthe plasma taking advantage of the 1st region and the 2nd region,thereby manufacturing the semiconductor devices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view for illustrating an ion current density measuringinstrument according to an embodiment of the present invention;

FIG. 2 is an A—A cross-sectional view of FIG. 1;

FIG. 4 is an enlarged cross-sectional view of a dashed line portion inFIG. 2;

FIG. 5 is a plan view for illustrating the manner in which the ioncurrent density measuring instruments according,to another embodiment ofthe present invention are located on a wafer;

FIG. 6 is a flowchart diagram for showing an ion current densitymeasuring method in an etching apparatus according to still anotherembodiment of the present invention;

FIG. 7 is a top view for illustrating an ion current density measuringinstrument according to still another embodiment of the presentinvention; and

FIG. 8 is a B—B cross-sectional view of FIG. 7.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, referring to the drawings, the explanation will be givenconcerning the embodiments of the present invention.

FIG. 1 illustrates an ion current density measuring instrument formed ona wafer. In the drawings, a silicon 1 is provided as a semiconductor ora conductor provided on an insulator 6. A silicon oxide film 2 is formedthereon as an insulator having a region the thickness of which isseveral n m or less locally. In a region on the silicon 1 which is incontact with the region in the silicon oxide film 2 the thickness ofwhich is thin, the impurity-concentration is maintained at a low value.Meanwhile, in the other region on the silicon, theimpurity-concentration is made higher by impurity implantation or thelike, thereby making the resistance ratio smaller. It is also allowablethat the region on the silicon 1 which is in contact with the thinregion in the silicon oxide film 2 is formed with a lowimpurity-concentration semiconductor, and the other region is formedwith a different conductor material.

A conductor film 4 is formed on the silicon oxide film 2 as a 2ndconductor. It is preferable to employ, as the conductor film 4, a highconductivity metal such as aluminum or copper, a polysilicon film dopedwith an impurity in a high concentration, or the like.

The structure in which, in this way, the conductor such as a metal, thesilicon oxide film, and the silicon are multilayered from the top isreferred to as “MOS (Metal Oxide Silicon) capacitor”. On the conductorfilm 4, insulators 5 having a W-wide and H-high substantiallyrectangle-shaped cross-section are formed with a constant spacingtherebetween. An edge portion and a side wall of the conductor film 4are covered with the insulators 5. A region on the conductor film 4exposed onto the surface is limited only to W-wide regions that aresandwiched between the insulators 5. On account of this, a solid angleformed from the surface of this 2nd region is made smaller than a 1stregion from the surface of which a substantially total solid angle isformed, the 2nd conductor 4 provided on the insulator 2 having the 2ndregion, one of the semiconductor or the conductor 1 and the 2ndconductor 4 having the 1st region.

It is preferable to form the insulators 5 so that the followingconditions are satisfied: Using a photoresist, a silicon oxide film, asilicon nitride film, or the like, the configuration of the 2nd regionbecomes a rectangle-shaped configuration. Simultaneously, the length ofthe shorter side falls in the range of {fraction (1/10)}th to ½th of theheight so that H/W≧2 can be maintained.

In general, the structure in which the insulators 5 are provided on anupper electrode of the MOS capacitor is referred to as “shadestructure-attached MOS capacitor”. The silicon 1 that has a structurelike this on its upper portion is located on the insulator 6 such as asilicon oxide film. In order to form on the insulator 6 the structure asdescribed above, it is desirable to employ, for example, a SOI wafer.

The shade structure-attached MOS capacitor is formed as follows: Before,halfway under, or after the formation of the above-described structure,a silicon situated on one side of the SOI wafer is etched in anisland-like configuration. Then, a semiconductor region that hasremained after the etching is cut off electrically from the surroundingsemiconductor region.

Next, the wafer equipped with the shade structure-attached MOS capacitoris set at a predetermined position in a plasma processing apparatus,then being exposed to the plasma.

In plasma etching or plasma-enhanced chemical vapour deposition used fora semiconductor device manufacturing a bias voltage for leading thepositively charged ions into the wafer is often applied to the wafer.This condition increases wafer-perpendicular velocity components of thepositively charged ions, causing the ions of the wafer-perpendicularvelocity components to be launched into the wafer with a tremendouslylarge velocity distribution function. On the other hand, since the massof the electrons is small, the electrons are not influenced so much bythe bias voltage, thus travelling in an isotropic manner.

When the wafer equipped with the shade structure-attached MOS capacitoris exposed to the above-described plasma, at a position from which theplasma forms a substantially total solid angle, such as the region onthe silicon 1 exposed onto the surface or the upper surfaces of theinsulators 5, the electrons and the ions reach the region or thesurfaces by the same flux on average. However, from the conductor film 4that is exposed onto the surface in a state of being surrounded by theinsulators 5, the plasma forms only a smaller solid angle. As a result,the electrons and the ions have a high probability of travellingperpendicularly to the silicon 1. Accordingly, the flux of the highkinetic-energy ions exhibits an isotropic behavior, thus becoming largerthan the flux of the low kinetic-energy electrons. Consequently, theconductor 4 turns out to have a positive electric potential Vg withreference to the silicon 1.

The existence of the electric potential difference between the conductor4 and the silicon 1 causes an electric current to flow through theregion in the silicon oxide film 2 the thickness of which is thin. Thisis referred to as the current-voltage characteristic. Table 1 shows thecharacteristic (the measurement values) in the case where the thinsilicon oxide film is 4.5 nm thick and 30 μm² in area. This area is themask's area of a silicon nitride film used for forming the thin siliconoxide film by heat oxidation.

TABLE 1 current-voltage characteristic of silicon oxide film Vg: voltageapplied to silicon oxide film (V) through silicon oxide film Ig: currentflowing 0  1 × 10⁻¹² 4.0  1 × 10⁻¹² 4.5  5 × 10⁻¹² 5.0  1 × 10⁻¹⁰ 5.4 1× 10⁻⁹ 5.8 1 × 10⁻⁸ 6.2 1 × 10⁻⁷ 6.7 1 × 10⁻⁶ 7.0 3 × 10⁻⁶

As is shown in Table 1, in general, the current-voltage characteristicof the thin silicon oxide film is as follows: When the applied voltage Vis equal to about 5 V or lower, only an infinitesimal electric currentflows. If, however, a voltage higher than that is applied, the currentthat will flow is gradually increased with the increase of the appliedvoltage. When a voltage higher than about 7 V is applied, the flowingcurrent is increased steeply.

In the state of being exposed to the plasma, the conductor 4 has thepositive electric potential Vg with reference to the silicon 1. At thistime, the current-voltage characteristic of the silicon oxide film 2allows the current Ig corresponding to Vg to flow.

Assuming that the current density f of the ions that will reach thesilicon 1 is 1 mA/cm², and the conductor's area exposed onto the surfaceis S, and if Vg is, e.g., 5. 8 V, the current Ig becomes equal to 1×10⁻⁸A. This current Ig is equal to a difference between the positivecharges, i.e., the ions, and the negative charges, i.e., the electrons,both the ions and the electrons reaching, in the unit time, theconductor portion exposed onto the surface.

At this time, even if the flux of the ions directed to bottoms of thegrooves is considered to be equal to the flux of the ions directed tothe flat portions on the upper portions of the grooves, and even if theelectrons are considered not to reach the bottoms of the grooves,namely, even if the substantial flux of the ions directed to the groovebottoms is considered as its maximum value, the area S needed to obtainIg is given by

S=Ig / f  (1)

The substitution of Ig=1×10⁻⁸ A and f=1 mA/cm² gives S=1×10⁻⁵ cm² =1000μm². When representing, by Sg (=30 μ²), the area of the above-describedregion in the silicon oxide film 2 the thickness of which is thin, thisvalue of S is about 30 times as large as Sg.

When the conductor's area S that is exposed onto the surface is equal to3×10⁻³ cm², which is about 10000 times as large as the thin region'sarea Sg in the silicon oxide film 2, Ig is equal to 3×10⁻⁶ A and eventhis time, Vg is equal to only 7 V. Consequently, as long as S/Sg isabout 10000 or less, Vg becomes equal to an order of only 7 V.

At the time when Vg becomes equal to at most the order of only 7 V, thebehavior of the ions and the electrons becomes as follows:

The applied bias voltage of tens of volts or more accelerates the ions.On account of this, the voltage of at most 7 V exerts no influence uponthe behavior of the ions. Namely, the velocity components of the ionsperpendicular to the silicon 1 have been tremendously large from thebeginning, and thus the ions reach the conductor film with their orbitshardly bent by Vg. Also, the upper portions of the grooves' side wallshave become negatively charged with reference to the silicon 1 onaccount of the electrons that had already reached there. This conditionmakes it rather difficult for the electrons to reach the insides of thegrooves. Accordingly, it is possible to neglect the effect of being ledinto the groove bottoms the potential of which becomes at most the orderof only 7 V.

As the aspect ratio of the groove defined by the height-to-width (H/W)of the groove becomes larger, the condition of making it unlikely forthe electrons to reach the groove bottom becomes more influential andnoticeable. If all the planes constituting the groove are electricallyneutral, the ratio at which the isotropically moving electrons willreach the groove bottom can be determined by considering a solid angleformed from the groove bottom toward the outside of the groove.

The larger the aspect ratio becomes, the smaller this solid anglebecomes. In particular, if the aspect ratio is 2 or more, the flux ofthe electrons that will reach the groove bottom is 20% or less of theflux of the electrons that will reach a flat portion of the groove'supper portion. In order to decrease the flux of the electrons reachingthe groove bottom, it is advisable to increase the aspect ratio of thegroove, and what is more desirable is to make the aspect ratio 4 ormore. If the aspect ratio is made equal to 4 or more, the flux of theelectrons reaching the groove bottom becomes equal to 10% or less of theflux of the electrons reaching the flat portion of the groove's upperportion. However, making the aspect ratio larger than this does notdecrease the flux of the electrons very much, and also makes it ratherdifficult to form the groove structure. Also, if the upper portion ofthe groove's side wall becomes negatively charged, the flux of theelectrons reaching the groove bottom is decreased even further. On theother hand, the ions do not depend on the aspect ratio of the groovevery much.

As explained above, when the aspect ratio is equal to 2 or more, theelectrons hardly reach the conductor on the groove bottom andconversely, almost all of the ions reach the conductor thereon. At thistime, the total quantity of the electric charges that will reach, in 1second, the conductor film 4 exposed onto the surface becomes equal tothe electric current Ig that flows through the thin silicon oxide film2, which is given by

Ig=f×S  (2)

Moreover, as a characteristic of the thin silicon oxide film 2, thereexists an electric charge Q that flows through the thin silicon oxidefilm 2 and that is needed to cause the dielectric breakdown to occur. Ifan electric charge more than Q flows through the thin silicon oxide film2, the thin silicon oxide film exhibits a current-voltage characteristicthat differs from the current-voltage characteristic prior thereto. Thisstate is regarded as a state where the thin silicon oxide film 2 hasundergone the breakdown. This current-voltage characteristic is therelationship between the current flowing between the silicon 1 and theconductor 4 and the voltage applied therebetween.

It is assumed that the thin silicon oxide film 2 has undergone thebreakdown after it has been exposed to the plasma during a time T. Theexpression of this process is given by the following equation:

 Q=Ig×T  (3)

Using the equation (2) and the equation (3), this equation can bemodified into

f=Q/S/T  (4)

All the variables at the right side of this equation can be checked anddetermined, and thus the ion current density f is measured using thesevalues.

If the following relationship or the like exists between an accurate ioncurrent density f′ that has been determined under a certain condition byanother method and the ion current density f that has been calculatedusing the present invention, it is also allowable to determine a moreaccurate ion current density f by converting f using the equation (4′):

f′=a×f+b  (4′)

Q can be calculated as follows: A shade structure-attached MOS capacitorthat is the same as the one to be exposed to the plasma is prepared.Next, the voltage is applied to its thin silicon oxide film 2, thenmeasuring an electric current that will flow through the thin siliconoxide film 2 at that time. Moreover, Q is calculated from the electriccurrent and the time T elapsing from the voltage application to the gatebreakdown. S, which is the area of the conductor 4 exposed onto thesurface, can be determined in advance from the design value, a SEMphotograph, or the like. T can be measured, since T is the time elapsinguntil the thin silicon oxide film 2 has undergone the breakdown by theexposure of the shade structure-attached MOS capacitor to the plasma.

By locating, on a wafer and in plural number, the above-described ioncurrent density measuring instruments having different Q or S, itbecomes possible to determine f and its distribution at a fewer numberof measurement times. Referring to FIG. 5, the example will be explainedbelow. In this example, the ion current density measuring instrumentshaving three types of different S (i.e., S1, S2, S3) are located on apiece of wafer. The three ion current density measuring instruments arelocated on the one silicon 1.

The plurality of instruments having the already-explained structure andthe different S have been prepared on the one piece of wafer. Thiscondition allows a critical value Scr to be measured at a single time,thus making it possible to measure the ion current density f or itsdistribution on the wafer. Here, the critical value Scr is a value onwhich the gate of S less than the critical value Scr will not be brokendown but the gate of S larger than that will be broken down.

The plurality of instruments having different Q instead of the differentS are prepared in the wafer, which also makes it possible to execute thesimilar measurements. It is possible to configure the thin silicon oxidefilms having the different Q by forming the thin silicon oxide filmshaving different thickness or areas Sg or by changing the formingcondition of the thin silicon oxide films. For the purpose of this, itis advisable to modify the time, the temperature, or the mask patternfor forming the thin silicon oxide films.

Furthermore, it is also allowable to measure the ion current density for its distribution on the wafer by, although the number of measurementtimes is increased, changing the time during which the ion currentdensity measuring instrument will be exposed to the plasma. Namely, themeasurements similar to the above-described ones are executed in pluralnumber, and then the measurement values are subjected to a statisticalprocessing, thereby determining Q, T, and f. This makes it possible toenhance the reliability of the measurements.

Concerning a thin silicon oxide film that has not been broken down evenif it has been exposed to the plasma during the certain fixed time T,the ion current density f can be measured as follows: The voltage isapplied to the thin silicon oxide film 2 in a state of being not exposedto the plasma, then measuring an electric current that will flow throughthe thin silicon oxide film 2 at that time. Next, the thin silicon oxidefilm is broken down, then calculating an electric charge Q′ that hasflown through the thin silicon oxide film 2 and that is needed for thegate breakdown in the state of being not exposed to the plasma. Finally,using Q′, the ion current density f can be measured by

f=(Q−Q′)/S/T  (5)

For this equation to be held, the electric potential of the silicon 1must be in a state of being influenced by only the plasma in closeproximity thereto. Even when the plasma is not uniform, the electricpotential of the silicon 1 is implemented so that it must not beinfluenced by the plasma positioned at a far distance.

Assuming that the electric potential of the silicon 1 has beeninfluenced by the plasma positioned at the far distance, there exists apossibility that, even if no shade structure exists, a difference in theelectric potential occurs between the conductor 4 on the upper portionof the MOS capacitor and the silicon 1. Then, the existence of theelectric potential difference between the conductor 4 and the silicon 1causes an electric current corresponding to the electric potentialdifference to flow through the thin silicon oxide film 2. As a result,it turns out that the electric current flowing through the thin siliconoxide film 2 is not determined by a difference between the currentdensities flowing into the silicon 1 and the conductor 4 that sandwichthe thin silicon oxide film.

Thus, it is advisable to restrict the silicon 1 and the conductor 4 to arange where the state of the plasma can be regarded as substantiallyunchanged from the locally thin region in the silicon oxide film 2.

In addition, the insulator 6 such as a silicon oxide film is located inthe surroundings of the silicon 1 exposed onto the surface, therebyelectrically isolating the silicon 1 from the other regions. As aconsequence, even when the plasma is not uniform, the electric potentialof the silicon 1 is determined by the influence of the plasma in closeproximity thereto.

For electrically isolating the silicon 1 of the shade structure-attachedMOS capacitor, the following methods are preferable in addition to themethod using the SOI wafer: A method of pasting the shadestructure-attached MOS capacitor onto an insulator, a method of etchingthe silicon 1 after pasting onto an insulator the shadestructure-attached MOS capacitor where the silicon 1 is not electricallyisolated, or the like.

Next, referring to FIG. 7 and FIG. 8, the explanation will be givenbelow concerning another embodiment. What differs from the embodimentillustrated in FIG. 1 is that a silicon nitride film 3 is sandwichedbetween the silicon oxide film 2, which has the region in which thethickness of the film has been made locally thin, and the conductor film4.

The structure like this is referred to as “MNOS (Metal Nitride OxisideSilicon) capacitor”. When the voltage Vg is applied to the MNOScapacitor, electrons or positive holes are injected into the capacitorfrom the silicon 1 in correspondence with the voltage, and as aconsequence, a flat band voltage Vfb is shifted. Also, in the MNOScapacitor as well, the current-voltage characteristic exists which isquite similar to that in the MOS capacitor, and thus the electriccurrent Ig flows in correspondence with the applied voltage Vg.

The shade structure as explained in FIG. 1 is provided on the conductor4 of the MNOS capacitor, thereby forming a shade structure-attached MNOScapacitor. The exposure of this shade structure-attached MNOS capacitorto the plasma causes the conductor 4 to have the positive electricpotential Vg with reference to the silicon 1, and thus the MNOScapacitor traps the electrons in response to the positive electricpotential Vg. Measuring the shift quantity (Vfb) of the flat bandvoltage results in a positive value. Investigating the relationshipbetween Vfb and Vg in advance makes it possible to determine, from Vfbthat has been measured after the exposure to the plasma, the electricvoltage Vg that was being applied to the thin silicon oxide film 2during the exposure to the plasma. Also, the MNOS capacitor'scurrent-voltage characteristic investigated in advance makes it possibleto determine the electric current Ig that will flow when the electricvoltage Vg is applied. As described previously, Ig is equal to theproduct of the current density f of the ions and the conductor's area Sexposed onto the surface. Thus, f is given by

f=Ig/S  (6)

and the ion current density f can be determined from Ig and S. In orderto investigate a variation in the flat band voltage, thecapacitance-voltage characteristic is measured before and after theexposure to the plasma. Here, the capacitance refers to a capacitance ofthe thin silicon oxide film, and the voltage refers to a direct voltageapplied to the thin silicon oxide film.

A direct voltage Vd of an order of −10 V to +10 V is applied to thesilicon oxide film 2 and the silicon nitride film 3 and in addition, ahigh-frequency voltage of an order of 1 MHz is further applied thereto,thereby investigating the relationship between the capacitance C and theapplied voltage Vd. Next, after applying a fixed direct voltage Vgduring the same time T as the time of the exposure to the plasma, therelationship between the capacitance C and the applied voltage Vd isinvestigated by much the same method. At this time, before and afterapplying the fixed direct voltage Vg during the fixed time T, the shiftquantity (Vfb) of the flat band voltage for obtaining a certain fixed Ccan be derived from a variation in Vd. Determining this Vfb as afunction of T and Vg in advance makes it possible to recognize at abouthow much electric potential the conductor 4 has been caused to be withreference to the silicon 1 by the plasma. This is done by determiningVfb that has been generated after the exposure to the plasma during thefixed time T. Also, the current-voltage characteristic investigated inadvance otherwise allows the voltage to be converted into an electriccurrent, thereby making it possible to calculate the electric current I.Accordingly, using the conductor's area S exposed onto the surface, theion current density f can be determined by the equation (6).

In general, in the above-described relationship between the appliedvoltage Vg and the shift quantity Vfb of the flat band voltage, thereexists a region where, even if Vg is varied, Vfb is not varied. Theregion like this is referred to as “dead band”. In the case where, e.g.,when measuring the ion current density using the shadestructure-attached MNOS capacitor, Vg wished to be utilized is includedin the dead band of Vfb, a voltage is applied to the MNOS capacitorbeforehand so as to shift the Vg-Vfb characteristic. This allows theshade structure-attached MNOS capacitor to be operated in a region otherthan the dead band of Vfb, thereby making it possible to measure the ioncurrent density f.

Instead of using the shade structure-attached MOS capacitor or the shadestructure-attached MNOS capacitor, the ion current density f can also bemeasured using a nonvolatile storage apparatus such as a shadestructure-attached EEPROM. Taking an EEPROM as the example, anonvolatile memory such as the EEPROM is formed on an insulator, andthen the shade structure as illustrated in FIG. 1 is formed on itscontrol gate electrode with an insulating material. Meanwhile, afterforming a structure in which a portion of the substrate of thenonvolatile memory such as the EEPROM is exposed onto the surface, thestructure is exposed to the plasma. This causes the electric potentialdifference Vg to occur between the control gate electrode toward whichthe reaching of the electrons is suppressed and the silicon toward whichthe reaching of the electrons is not suppressed. The EEPROM stores thiselectric potential difference as the shift quantity Vth of a thresholdvalue voltage. It is possible to investigate the relationship between Vgand Vth in advance. Thus, after the process has been terminated, themeasurement of Vth allows Vg to be calculated. Accordingly, the ioncurrent density f can be calculated from the relationship between Vg andIg flowing through the gate.

Although, in the above-described explanation, the shade structure isformed on the conductor on the thin silicon oxide film, the formation ofthe shade structure on the silicon 1 also makes it possible to performthe same measurements and calculations.

The ion current density measuring instrument explained so far can beconfigured using only the materials used for the semiconductor devices.This condition, unlike the general probe, results in no worry about themetal contamination. Also, since the electrical wiring need not beinstalled therethrough, it is possible to measure the ion currentdensity without making the special contrivance to the plasma processingapparatus.

Unlike the ion current density measuring instrument the representativeof which is based on the conventional probe method, the ion currentdensity measuring instrument according to the present embodiment iseasily applicable to the mass-producing apparatus. In the massproduction of the semiconductor devices, the instrument is preferablefor determining the timing of executing a wet cleaning processing wheresuch an organic solvent as alcohol, pure water, or the like is used.Otherwise, the instrument is preferable for ascertaining whether or notthe apparatus has been restored back to its original state when theapparatus is assembled after the wet cleaning.

Namely, as is indicated in FIG. 6, it is advisable to beforehand measurean ion current density distribution s 0 under a predetermined conditionat the time when a desirable etching result can be obtained in anetching apparatus or a desirable CVD result can be obtained in a CVDapparatus.

Repeating the plasma processing in the etching apparatus or the CVDapparatus makes the following unavoidable: A film is deposited on theinner wall or the electrode of the etching apparatus or the CVDapparatus, the inner wall or the electrode is shaved, or the surface ofthe inner wall is deteriorated in quality.

The film that has been deposited on the inner wall exerts influencesupon the etching or the CVD. The influences exerted upon the etching orthe CVD, in many cases, result in the necessity for destroying themass-produced products to check them.

Thus, using the ion current density measuring instrument according tothe present embodiment under the above-described predeterminedcondition, the ion current density distribution is measured periodicallywhile the mass-producing process is repeated several times.

Based on this, as compared with a result measured when the etchingresult or the CVD result was obtained and becoming a criterion, if thedistribution coincides with the result within a fixed range and lieswithin a suitable region, the mass production is continued. If thedistribution does not coincide therewith, the apparatus is checked, thenexecuting the wet cleaning of the components where such an organicsolvent as alcohol, pure water, or the like is used, replacements of thecomponents, or the other adjustments. After that, the ion currentdensity distribution is measured using the ion current density measuringinstrument according to the present embodiment again. Then, afterconfirming that the result coincides with the criterion value within thefixed range, the mass production is restarted.

The above-described method allows the wet cleaning frequency to besuppressed down to the smallest possible degree. Moreover, the methodmakes it possible to suppress the occurrence of a defective product whenthe mass production is restarted with the apparatus after the wetcleaning, thereby allowing the operation ratio of the apparatus to beenhanced substantially.

The condition under which the ion current density is measured may differfrom a condition employed in the actual mass production. Thus, thecondition may be a pressure made higher by the adjustment of a valvethan the pressure within the apparatus used for the mass production, orthe ion current density may be measured using a gas differing from theone used in the actual mass production. Also, the employment of a gasthat does not cause the etching or the film's deposition makes itpossible to use the ion current density measuring instrument repeatedly.

Moreover, by estimating a cause of the yield decrease in the massproduction, it is possible to eliminate the cause. For example, when theforeign substances are produced so frequently and the frequentproduction is attributed to an abnormal electric discharge at a certainposition within the plasma processing chamber, the use of the ioncurrent density measuring instrument as presented by the presentembodiment allows the ion current density distribution to be measured onthe wafer. By making a comparison between the ion current densitydistribution on the wafer in the state where the large number of foreignsubstances are produced and the ion current density distribution on thewafer in the normal state, it becomes possible to confirm the state ofthe apparatus. In addition, by focusing a special attention onto aproximity to a position where there exists a significant differencebetween the both so as to investigate the cause of bringing about theabnormal state, it becomes possible to restore the apparatus back to thenormal state in a short while.

In the plasma apparatus used for manufacturing the semiconductors, thereare many cases where, in order to accelerate the wafer processing, thebias voltage is applied so as to lead the ions into the wafer. In thestate where the bias has been applied to the wafer, when trying to usethe probe method so as to measure the ion current density reaching thewafer, the difficulty in the probe isolation results in the difficultyin the probe insertion. Accordingly, measuring the ion current densityby the probe method is substantially difficult.

The use of the ion current density measuring instrument as presented bythe present embodiment permits the ion current density to be measured inthe state where the bias has been applied to the wafer. This makes itpossible to use the instrument for the development of the plasmaprocessing apparatus or the processing process as well.

When optimizing the bias voltage to be applied to the wafer in theplasma processing apparatus, conventionally, the judgement could nothelp being made in accordance with the rate of the etching or the CVD,the cross-section configuration at the time when the pattern-attachedwafer has been processed, or the yield of the semiconductor devices.However, the investigation of the relationship between the voltage to beapplied and the ion current density reaching the wafer makes it easy tooptimize the bias voltage.

Also, the investigation of the ion current density distribution makes itpossible to optimize the apparatus's configuration, e.g., to determinethe distance from the wafer's edge to the inner wall of the processingchamber so that the ion current density distribution becomes a desirableone, or makes it possible to adjust the flow quantity, the pressure, orthe like.

What is claimed is:
 1. An ion current density measuring instrument forsetting a wafer at a predetermined position in a plasma processingapparatus so as to measure an ion current density at the time when saidwafer is exposed to a plasma, said ion current density measuringinstrument comprising said wafer which includes: a semiconductor or aconductor provided on an insulator, an insulator formed on saidsemiconductor or said conductor and having a region the thickness ofwhich has been made locally thin, a 2nd conductor provided on saidinsulator, a 1st region which one of said semiconductor or saidconductor or said 2nd conductor forms a substantially whole solid angleis formed, and a 2nd region which the other of said semiconductor orsaid conductor or said 2nd conductor forms a smaller solid angle thansaid 1st region, wherein said ion current density is determined bymeasuring an electric current flowing through said region in saidinsulator the thickness of which has been made locally thin.
 2. The ioncurrent density measuring instrument as claimed in claim 1, wherein anitride film is provided between said insulator and said 2nd conductor.3. The ion current density measuring instrument as claimed in claim 1,wherein there is provided said insulator formed so that configuration ofsaid 2nd region becomes rectangle-shaped configuration and length of ashorter side thereof falls in a range of ¼th to ½th of height thereof.4. The ion current density measuring instrument as claimed in claim 1,wherein said insulators or said 2nd conductors are located on saidwafer, electric charges needed to cause said insulators to undergodielectric breakdowns being different from each other, areas exposedonto the surfaces of said 2nd conductors being different from eachother.
 5. An ion current density measuring instrument comprising: meansfor exposing a wafer to a plasma, said wafer including a semiconductoror a conductor, an insulator formed on said semiconductor or saidconductor and having a region the thickness of which has been madelocally thin, and a second conductor provided on said insulator, one ofsaid semiconductor or said conductor and said second conductor having asecond region a solid angle formed from the surface of which is madesmaller than another region, and means for measuring an ion currentdensity of said plasma.